1. Field of the Invention
The present invention relates to the evaluation of a subterranean formation. In particular, the present invention relates to sampling fluid from a subterranean formation via a downhole tool positioned in a wellbore.
2. Description of the Related Art
The collection and sampling of underground fluids contained in subterranean formations is well known. In the petroleum exploration and recovery industries, for example, samples of formation fluids are collected and analyzed for various purposes, such as to determine the existence, composition and producibility of subterranean hydrocarbon fluid reservoirs. This aspect of the exploration and recovery process can be crucial in developing exploitation strategies and impacts significant financial expenditures and savings.
To conduct valid fluid analysis, the formation fluid obtained from the subterranean formation should possess sufficient purity, or be “virgin” or “clean” fluid, to adequately represent the fluid contained in the formation. In other words, the subterranean fluid is pure, pristine, connate, uncontaminated or otherwise considered in the fluid sampling and analysis field to be sufficiently or acceptably representative of a given formation for valid hydrocarbon sampling and/or evaluation.
Various challenges may arise in the process of obtaining clean fluid from subterranean formations. Again with reference to the petroleum-related industries, for example, the earth around the borehole from which fluid samples are sought typically contains contaminates, such as filtrate from the mud utilized in drilling the borehole. This so-called “contaminated fluid” often contaminates the clean fluid as it passes through the borehole, resulting in fluid that is generally unacceptable for hydrocarbon fluid sampling and/or evaluation. Because formation fluid passes through the borehole, mudcake, cement and/or other layers during the sampling process, it is often difficult to avoid contamination of the fluid sample as it flows from the formation and into a downhole tool. A challenge thus lies in minimizing the contamination of the clean fluid during fluid extraction from the formation.
FIG. 1 depicts a subterranean formation 3 penetrated by a wellbore 4. A layer of mud cake 5 lines a sidewall 7 of the wellbore 4. Due to invasion of mud filtrate into the formation during drilling, the wellbore is surrounded by a layer known as the invaded zone 9 containing contaminated fluid that may or may not be mixed with clean fluid. Beyond the sidewall of the wellbore and surrounding contaminated fluid, clean fluid is located in a portion of the formation 6 referred to as the connate fluid zone 8. As shown in FIG. 1, contaminates tend to be located near the wellbore wall in the invaded zone 9. Clean fluid tends to be located past the invaded zone and in the connate fluid zone 8.
FIG. 2 shows the typical flow patterns of the formation fluid as it passes from subterranean formation 3 into a downhole tool 1. Examples of a downhole sampling tool are disclosed in U.S. Pat. Nos. 4,860,581 and 4,936,139, both assigned to the assignee of the present invention. The downhole tool 1 is positioned adjacent the formation and a probe 2 is extended from the downhole tool through the mudcake 5 to the sidewall 7 of the wellbore 4. The probe 2 is placed in fluid communication with the formation 3 so that formation fluid may be passed into the downhole tool 1. Initially, as shown in FIG. 1, the invaded zone 9 surrounds the sidewall 7 and contains contamination. As fluid initially passes into the probe 2, the contaminated fluid from the invaded zone 9 is drawn into the probe with the fluid thereby generating fluid unsuitable for sampling. However, as shown in FIG. 2, after a certain amount of fluid passes through the probe 2, the clean fluid breaks through and begins entering the probe. In other words, a portion of the fluid flowing into the probe gives way to the clean fluid, while at least a portion of the remaining portion of the fluid may be contaminated fluid from the invaded zone. The challenge remains in capturing the clean fluid in the downhole tool without contamination.
Various methods and devices have been proposed for obtaining subterranean fluids for sampling and evaluation. For example, U.S. Pat. No. 6,230,557 to Ciglenec et al., U.S. Pat. No. 6,223,822 to Jones, U.S. Pat. No. 4,416,152 to Wilson, U.S. Pat. No. 3,611,799 to Davis and International Pat. App. Pub. No. WO 96/30628 have developed certain probes and related techniques to improve sampling. Other techniques have been developed to separate clean fluids during sampling. For example, U.S. Pat. No. 6,301,959 to Hrametz et al. discloses a sampling probe with two hydraulic lines to recover formation fluids from two zones in the borehole. Borehole fluids are drawn into a guard zone separate from fluids drawn into a probe zone. Despite such advances in sampling, there remains a need to develop techniques for fluid sampling to optimize the quality of the sample and efficiency of the sampling process.
Various techniques have also been employed for perforating the sidewall of a wellbore and sampling therethrough. For example, U.S. Pat. No. 5,692,565 assigned to the assignee of the present invention discloses techniques for perforating the sidewall of a cased wellbore using a downhole tool with a flexible drilling shaft. Other techniques, such as those in U.S. Pat. No. 5,195,588 assigned to the assignee of the present invention, disclose the use of punching mechanisms, explosive devices and/or other tools for creating a perforation into the sidewall of a wellbore for sampling. While these techniques provide the ability to create perforations into the sidewall of the wellbore, there remains a need to sample clean fluid through the perforation.
In considering existing technology for the collection of subterranean fluids for sampling and evaluation, it is desirable to have a downhole sampling tool capable of providing one or more, among others, of the following attributes: the ability to sample with reduced contamination, selectively collect clean fluid apart from contaminated fluid, optimize the quantity of clean fluid captured, reduce the amount of time it takes to obtain clean formation samples, reduce the likelihood of contamination from fluids in the invaded zone and/or wellbore and improve the quality of clean fluid extracted from the formation for sampling. To this end, the present invention is provided.